The invention is in the field of ionized gas plasma torches used for example in conjunction with an optical spectrometer or with a mass spectrometer for the purpose of elemental analysis and specifically to torches having replaceable parts.
Plasma elemental analysis is an important branch of chemical analysis. The plasma is generated in a device called a torch (or burner) and a sample is introduced into the plasma so that elements of the sample are atomized by the plasma and detected by a number of techniques. Most torches comprise three concentric tubes. The outer tube contains the plasma and is generally a quartz tube which can withstand the relatively high temperatures to which it is exposed. The intermediate tube is positioned concentric within the outer tube, and terminates within the outer tube and also is generally a quartz tube. A flow of plasma gas, such as argon, is flowed in the intermediate tube and the plasma is generated for example by inductive coupling of radio frequency energy to the ionized gas so that the resulting plasma is above the intermediate tube and within the outer tube. The inner tube is concentric within the intermediate tube and also terminates within the outer tube so that a sample, generally in the form of an aerosol, can be flowed in the inner tube and then into the plasma. The heat of the plasma can melt the outer tube and to prevent this coolant gas, generally argon, is flowed in the annulus between the outer tube and the intermediate tube and at a relatively high velocity so that the plasma is kept away from the inner surface of the outer tube and to cool the outer tube. The flow of gas in this annulus and the annulus between the intermediate tube and the inner tube is conventionally helical in practice by introducing the gases to the tubes tangentially to the axis of the tubes. The position of the intermediate tube with respect to the other tubes is critical. Ideally, the tubes are exactly concentric so that the plasma is concentric in the torch. In practice, some tolerance in concentricity is allowable as long as the plasma is essentially concentric in the torch as is well understood in the art. In addition, the inner tube should also be concentric in the intermediate tube so that the sample is introduced into the center of the plasma under well defined conditions. Due to the larger annulus area between the inner tube and the intermediate tube than between the outer tube and the intermediate tube, less precision is required with the concentricity of the inner tube in the intermediate tube than with the concentricity of the intermediate tube in the outer tube (although near perfect concentricity is always most preferred).
Many torches are constructed by glass blowers entirely of quartz tubing. The glass blower carefully aligns the tubes so that they are essentially concentric during fabrication and thereafter the tubes remain frozen in alignment. However, a problem with this type of torch occurs when one of the tubes is damaged or deteriorates with use, for example by the devitrification of the intermediate tube, limiting its analytical utility. Then the whole torch must be removed from service and the torch discarded or it can be repaired by a glass blower, at a cost typically greater than the original price of the torch. In addition, with this type of torch the experimenter is locked into a fixed configuration of the torch and is unable to conveniently alter the shape or size of any of the tubes of the torch for the purposes of optimizing its use. Demountable torches were developed to overcome these problems by removably mounting the tubes in a base.
A problem with prior demountable torches was the need to align the intermediate and outer tubes after replacing one or the other. With the Perkin Elmer Demountable Torch, for example, initial alignment is accomplished with a tool that is inserted in the annulus between the outer and intermediate tubes while adjusting three screws that bear radially on the outer tube. This initial adjustment is then fine tuned during use of the torch, if necessary, by further adjustment screw turning to obtain an essentially concentric plasma in the torch. This is often time consuming and skill intensive.
A solution to the alignment problem in demountable torches was the demountable torch described by Windsor et al. in Applied Spectroscopy, Vol. 33, 1979, on pages 56-58, wherein the outer and intermediate tubes were aligned by a series of spacers, inserts, 0-rings and collars. Windsor et al stated on page 57 "Precise alignment of the glass-blown torch can be achieved during construction through the use of an assembly jig. However, this type of torch is not easily repaired should problems be encountered. The dismantable base approach can be difficult to align if the three tubes are held only near the bottom. This problem is aggravated by the fact that rarely is commercial quartz and Pyrex glass tubing perfectly straight or cylindrical. A torch design which is both dismantable and self-aligning has been used successfully in this laboratory for several years. A unique feature is the use of nylon or Teflon slip fit spacers between the sample and plasma gas tubes and the plasma and coolant gas tubes." While this torch may have solved the alignment problem it was also composed of over 20 parts. It would be more desirable if a demountable pre-aligned torch were composed of fewer parts, and its success in use be more independent of operator skill.
Semi-demountable torches are torches having fixed outer and intermediate tubes and a replaceable inner tube. The Allied Systems ICP torch is an example of a semi-demountable torch which has a boron nitride base onto which the outer and intermediate tubes are permanently fixed while the inner tube is removably positioned in the base.
The present invention is a demountable torch comprising pre-aligned intermediate and outer tubes, as few as three parts and no base. This is accomplished by the use of tubes having precision aligned conical joints, preferably the well known standard taper precision ground quartz type joint.